The invention relates to a method for producing, from a metal sheet, a hollow profile which is longitudinally slotted, in particular as a pre-product of a longitudinal seam welded hollow profile, which consists of several longitudinal segments having different cross sections, at least one segment of which becomes larger or smaller along its length.
Hollow profiles of this kind can be produced by various ways and means. In the case of a first production process, on which the invention is based, the hollow profile is welded together from individual cut metal pieces. Thus a hollow profile with a cylindrical longitudinal segment of small diameter, an adjoining conical longitudinal segment and a cylindrical longitudinal segment of larger diameter joining thereon has to be precise all along a constant wall thickness, but its production is complex because of the longitudinal segments to be cut individually, then to be formed and finally to be welded together. In addition the weld seams form places of irregularities, which are disadvantageous as regards further reworking by shaping, for example by internal high pressure forming, and starting points for crack formation during further shaping and later operational use.
In the case of another known production process the hollow profile is produced from one piece of cut metal sheet. In order to obtain different cross sections in individual longitudinal segments, the hollow profile can be expanded by internal high pressure forming. Should one wish to produce a hollow profile shape of the example described, the cross section expansion would be totally at the expense of the wall thickness. Therefore this production process is only suitable to a limited extent.
Finally a two-stage method for producing a hollow profile with flange is known (DE 199 05 365 A1), wherein in a first stage an otherwise rectangular hollow profile with a hook-shaped flange and a stepped recess is produced. In order to increase the cross section of this hollow profile, it is expanded in a subsequent stage by internal high pressure and as a result attains its final form.
The aforementioned problem of wall thickness change during internal high pressure forming due to different cross sections in individual longitudinal segments of the hollow profile is not solved by this prior art.